Method of forming high capacitive-coupling ratio and high speed flash memories with a textured tunnel oxide

1. A method for manufacturing a nonvolatile memory on a semiconductor substrate, said method comprising the steps of: forming a gate dielectric layer on said semiconductor substrate; forming a first polysilicon layer on said gate dielectric layer; forming a silicon nitride layer on said first polysilicon layer; patterning said silicon nitride layer, said first polysilicon layer and said gate dielectric layer to form a gate structure on said semiconductor substrate; performed a first thermal oxidation to form a first oxide layer on said semiconductor substrate exposed by said gate structure and simultaneously form a polyoxide on side walls of said gate structure; forming isolating spacers on side walls of said gate structure; performing an ion implantation to form source and drain of said nonvolatile memory in said semiconductor substrate; performing a second thermal oxidation to form a second oxide layer on said semiconductor substrate exposed by said gate structure and said isolating spacers, wherein the dopants of said first ion implantation being driven into said semiconductor substrate; removing said silicon nitride layer, said isolating spacers, then removing said polyoxide layer and said gate oxide that is uncovered by said gate structure; forming undoped silicon layer or hemispherical grained silicon along a surface of said gate structure and said second oxide layer; performing a third thermal oxidation to convert said undoped silicon layer or said hemispherical grained silicon into a third oxide layer having textured interface between said semiconductor substrate and said third oxide layer; removing said third oxide layer, wherein said gate structure has a rugged surface after said third oxide layer is removed; forming a fourth oxide layer on said gate structure and on said substrate; forming a second polysilicon layer on said fourth oxide layer; anisotropically etching said second polysilicon layer, thereby forming polysilicon side wall spacers on said side walls of said gate structure, said fourth oxide layer at top of said gate structure and uncovered by said polysilicon side wall spacers being removed, said fourth oxide layer remaining under said polysilicon side wall spacers being used as a tunneling oxide; forming a third polysilicon layer on said gate structure, said polysilicon side wall spacers; patterning said third polysilicon layer to define a floating gate region, said first polysilicon layer, said polysilicon side wall spacers and said third polysilicon layer to being used as a floating gate; forming an inter poly dielectric layer on said floating gate; and forming a fourth polysilicon layer on said inter poly dielectric layer, wherein said fourth polysilicon layer is used as a control gate.

2. The method of claim 1, wherein said gate dielectric is composed of silicon oxide.

3. The method of claim 1, wherein said gate dielectric is composed of JVD nitride, wherein said JVD nitride is referred to a silicon nitride formed by using jet vapor deposition.

4. The method of claim 1, wherein said isolating spacers are composed of silicon nitride.

5. The method of claim 1, wherein said first thermal oxidation is performed in N.sub.2 O ambient.

6. The method of claim 1, wherein said first thermal oxidation is performed in NO ambient.

7. The method of claim 1, wherein said first thermal oxidation is performed in N.sub.2 and O.sub.2 ambient.

8. The method of claim 1, wherein said first thermal oxidation is performed at a temperature about 700 to 1150 degrees centigrade.

9. The method of claim 1, wherein said isolating spacers are removed by using hot phosphorus acid solution.

10. The method of claim 1, wherein said silicon nitride layer is removed by using hot phosphorus acid solution.

11. The method of claim 1, wherein said polyoxide layer is removed by using HF solution or BOE solution.

12. The method of claim 1, wherein said gate oxide is removed by using HF solution or BOE solution.

13. The method of claim 1, wherein said third thermal oxidation comprises dry oxidation.

14. The method of claim 13, wherein said third thermal oxidation is performed in O.sub.2 ambient.

15. The method of claim 13, wherein said third thermal oxidation is performed at a temperature about 700 to 1000 degrees centigrade.

16. The method of claim 1, wherein said undoped silicon layer comprises amorphous silicon.

17. The method of claim 16, wherein said amorphous silicon is formed at a temperature about 400 to 600 degrees centigrade.

18. The method of claim 16, wherein said amorphous silicon is formed in an ambient containing SiH.sub.4 /N.sub.2.

19. The method of claim 16, wherein said amorphous silicon is formed to have a thickness about 20 to 100 angstroms.